1. Field of the Invention
The present invention relates to novel polymeric binder compositions useful in the prototyping of metal and ceramic parts. Methods of binder preparation and processes for producing all-metal or all-ceramic prototype or test parts by laser sintering technology are also part of the invention.
2. Description of the Related Art
Rapid prototyping of parts for machinery and manufacturing processes is important in development and testing of improved designs and new fabrications. In general, a thermally fusible substrate, such as a wax or plastic, is shaped to a desired form.
In the case where the substrate is not a thermally fusible material, such as metal or ceramic, the substrate is mixed with a thermoplastic binder material. This substrate/binder is shaped to a desired form, which is known in the field as a green part. When the binder is removed, typically by heating, the resulting metal or ceramic prototype will maintain the basic shape characteristics of the green part. Additional post-processing steps may then be employed to increase the strength and modulus of the green part.
Several processes are available for the fabrication of solid objects (Bourell et al., 1990, Ashley, 1991), including the Soligen.TM. and Selective Laser Sintering.TM. (SLS.TM.) process (Deckard, 1986, Deckard, 1988).
Each may employ substrate powders combined with "fugitive" thermoplastic binders. The composites are shaped into component parts, then heated to remove the binder, thus producing a part free of binder material.
An efficient method for coating the inorganic particles with a binder is important for obtaining a green part with a high green strength (Vail and Barlow, 1991). Generally, this is accomplished if the binder is soluble in an easily vaporized carrier media. Water, as well as several organic solvents, can be used for this purpose (Masters, 1985). Removal of the binder subsequent to shaping of the green part must be accomplished with minimal residue; otherwise, the integrity of the substrate material may not be maintained. Many binders leave significant residues when vaporized and therefore are not suitable for preparation of accurately proportioned prototype parts obtained from green parts.
Some current methods of preparing parts from high temperature materials are described in U.S. Pat. No. 5,182,170. Several processes, including sandcasting and injection molding, have been used to produce solid objects from a three-dimensional model or mold. Another method for rapid preparation of thermoplastic parts and wax patterns for lost wax castings employs a laser to create a solid object from model parameters stored in a CAD data base. Green parts are produced by successive deposition and laser sintering of thin layers of thermo-fusible materials. A laser sintering method, known as SLS.TM. is described in U.S. Pat. Nos. 5,076,869, 5,076,869, 4,863,538, 5,017,753, and U.S. Pat. No. 4,938,816, all incorporated herein by reference. In commercial applications of SLS.TM., a low power, raster scanned modulated CO.sub.2 laser is employed to selectively sinter thermoplastic powders in accordance with computer information about the object cross-section from a data base.
However, the low laser power and consequently low achievable powder fusion temperatures employed in this commercial process prevent the use of existing technology for directly fusing metal and ceramic powders that have high softening, sintering or melting temperatures (Zong et al., 1992). Present laser sintering technology is limited to preparing parts from powdered waxes and thermoplastic materials such as nylon, polycarbonate ABS and the like as described in U.S. Pat. No. 5,156,697 and U.S. Pat. No. 5,147,587, incorporated herein by reference. Current technology has failed to provide suitable fugitive binders for use with ceramic and metal powders in laser sintering processes. Consequently, access to hard, durable metal or ceramic parts through rapid prototyping processes such as SLS.TM. is not available.
Therefore there exists a need to develop rapid prototyping methods for preparing metal, ceramic and ceramic-metal composite parts. Such methods would likely result in a significant lowering of design costs by comparison with less practical methods of making such parts for design and test purposes.